Several high alloy carburizing steels (HACS) have been developed for bearings and gears which operate under extreme conditions of temperature or corrosive environment. HACS alloys include CBS 1000M and CBS 50NiL made by the Latrobe Steel Company (a subsidiary of the Timken Company), Pyrowear 53 and Pyrowear 675 made by Carpenter Technology Corp. and Vasco X-2M (Teledyne Vasco). Nominal compositions of these alloys are listed in Table I.
HACS alloys are also used in aerospace applications such as helicopter gearing, and in drill bits for deep well drilling. Because these alloys resist softening at elevated temperatures, they are also useful for machine components that operate in less extreme environments, but which may be inadequately lubricated.
Unfortunately, for reasons that are not well understood, HACS components cannot be gas carburized in the as-machined condition, although they can be pack carburized or plasma carburized. METALS HANDBOOK, vol. 4, Tenth Edition, ASM International, 1991. Gas carburizing, pack carburizing and plasma carburizing are described in articles on pages 312-24, 325-328 and 352-62, respectively.
Gas carburizing is the method of choice for high volume production. Virtually all carburization of automotive components worldwide is done by gas carburizing. In a conventional gas carburizing atmosphere, the oxygen content is too low to allow iron oxide to form, but there is enough oxygen in the furnace atmosphere that oxides of alloying elements such as Cr, Si, Mn and V can form. Apparently, it is the formation of these alloy oxides on the metal surface in a gas carburizing atmosphere which prevents adequate pick-up of carbon.
TABLE I __________________________________________________________________________ NOMINAL COMPOSITIONS OF HACS ALLOYS PYROWEAR PYROWEAR VASCO X- ELEMENT CBS1000M CBS50NIL 53 675 2M __________________________________________________________________________ Carbon 0.13 0.13 0.11 0.07 0.14 Manganese 0.55 0.25 0.35 0.65 0.30 Silicon 0.50 0.20 0.80 0.40 0.90 Chromium 1.05 4.20 1.00 13.00 5.00 Nickel 3.00 3.40 2.25 2.60 Cobalt 5.40 Molybdenum 4.50 4.25 3.25 0.80 1.40 Vanadium 0.40 1.20 0.10 1.60 0.45 Copper 2.10 Tungsten 1.35 __________________________________________________________________________
U.S. Pat. No. 3,885,995 discloses that if components made of HACS alloys are heated in air at temperatures on the order of 900.degree.-1000.degree. C. for times of 30 minutes or more prior to gas carburizing, then the alloys will carburize satisfactorily. Typically, exposure to air at these temperatures forms a loosely adherent oxide layer (primarily FeO) over a mixed metal-oxide sublayer a few microns in thickness. The sublayer consists of fingers of oxide penetrating into the metal matrix. Metal is lost from the surface of parts and the surface is roughened during the preoxidation treatment. Therefore, parts which are preoxidized prior to carburization must be finish-machined in the hardened condition after heat treatment to achieve a smooth surface and precise control of dimensions.